The invention relates to an objective lens for focusing a diverging radiation beam into a diffraction-limited radiation spot, comprising a single lens body of transparent material having a first refractive surface at the object side and a second refractive surface at the image side, at least one of said surfaces being aspherical. The invention also relates to an optical scanning device provided with such a lens.
Such a lens and scanning device are known from U.S. Pat. No. 4,668,056. In the embodiment described in said Patent in which a diverging beam emitted by a radiation source is focused to a diffraction-limited radiation spot having a half-value width of the order of 1 .mu.m on an information plane of a transparent record carrier, the objective lens is a biconvex lens one of whose refractive surfaces is aspherical. An aspherical lens surface is understood to mean a surface whose fundamental shape is aspherical but whose real shape has small deviations therefrom so as to correct for spherical aberrations of the fundamental shape. The known lens has a relatively large diffraction-limited image field with a diameter of the order of 200 .mu.m so that a sharp radiation spot can also be formed in the surroundings of the optical axis enabling the position of the scanning spot with respect to a pattern of tracks to be scanned in the information plane to be corrected by displacing the objective lens and the scanning beam with respect to each other.
The build-in length, from the radiation source to the information plane, of the known device is roughly given by (M+.sub.M /.sup.1 +2).f+hh' in which M is the magnification, f is the focal length, i.e. the distance between the object focus and the object principal point, and hh' the distance between the main surfaces H and H' of the objective lens. For a thin lens the term hh' is negligible with respect to the left term. The build-in length of the said embodiment, in which the magnification is of the order of 4.5 and the focal length is of the order of 3.2 mm, is approximately 20 mm. To reduce the build-in length while ensuring the same magnification, the focal length should have to be reduced. However, problems may then occur in connection with the desired free working distance of the objective lens when using it for scanning an information plane in a record carrier. In fact, this information layer is located behind a transparent substrate having a given thickness, for example 1.2 mm, while the record carrier must be placed at some distance from the objective lens so as to prevent the objective lens from bumping against the record carrier and being damaged in the case of unwanted movements of the record carrier with respect to the objective lens.